Ring laser angular rate sensors, often also called ring laser gyros, are well known. One example of a ring laser angular rate sensor is U.S. Pat. No. 4,751,718 issued to Hanse, et al., which is incorporated herein by reference thereto. Present day ring laser angular rate sensors include a thermally and mechanically stable laser block having a plurality of formed cavities for enclosing a gap. Mirrors are placed at the extremities of the cavities for reflecting laser beams and providing an optical closed-looped path.
Associated with such sensors is an undesirable phenomenon called lock-in which has been recognized for some time in the prior art. In the prior art, the lock-in phenomenon has been addressed by rotationally oscillating such sensors. The rotational oscillation is typically provided by a dither motor. Dither motors of the prior art usually have a suspension system which includes, for example, an outer rim, a central hub member and a plurality of three dither motor reeds which project radially from the hub member and are connected between the hub member and the rim. Conventionally, a piezoelectric element which serves as an actuator is connected to the suspension system. The suspension system comprises a dither motor which causes the block of the sensor to oscillate angularly at the natural mechanical resident frequency of the suspension system. This dither motion is superimposed upon the actual rotation of the sensor in inertial space. Such dither motors may be used in connection with a single ring laser gyro, or to dither multiple ring laser gyros. The prior art includes various approaches to recover inertial rotation data free from dither effects.
Certain applications of such ring laser gyros require exposure to severe environments. For example, ring laser gyros may be employed in airborne vehicles or devices which are subjected to extremely high gravitational forces, also called g forces herein. Such high gravitational forces may be on the order of about 14,000 g's. Gravitational forces of such high magnitude subject the piezoelectric elements to extremely high stresses. Unfortunately, such piezoelectric elements are brittle and will fracture or depolarize when subjected to such extremely high stresses, causing the ring laser gyro to malfunction.
Dither motors in the prior art are typically directed toward dithering a single ring laser gyro with a single dedicated dither motor imbedded in the hub of each ring laser gyro. Examples of such systems include U.S. Pat. No. 4,653,918 to Stjern, et al. and U.S. Pat. No. 4,349,183 to Wirt, et al., as well as the Hanse '718 ring laser gyro.
Dither motors for dithering multiple ring laser gyro beams within an integral ring laser gyro block are also known. For example, U.S. Pat. No. 3,503,688 to LeChevalier, entitled "Multiple Axis Laser Angular Rate Sensor", discloses a solid block support apparatus which supports three sets of counter rotating laser beams traveling along triangular paths. A shaft and block are vibrated or dithered about the axis of the shaft by a single mechanical dithering means. Components of the dither are presented to each triangular path and all three of the triangular paths are simultaneously dithered.
U.S. Pat. No. 4,856,901 to Ferris, et al., entitled "Velocity Control System Using Piezoelectric Transducers", discloses a design strikingly similar to the dither concept used in the LeChevalier '688 patent. Three ring laser gyro paths integrated into a solid block are mechanically dithered by a single dither mechanism through a cube diagonal. The present invention, in contrast, is a device for simultaneously dithering three discrete ring laser gyros each of polygonal shape and does not require an integrated solid block construction as in the '688 patent. Thus, the present invention allows the construction of smaller ring laser gyro packages using discrete ring laser gyros. This avoids the complexities and expense of fabricating a single three gyro block.
Other art provides various dither schemes. U.S. Pat. No. 4,801,206 to Benoist, entitled "Simplified Ring Laser Gyroscope Dither Control and Method", is also directed to a dither mechanism. A dither flexor is mounted between a ring laser gyroscope body and a support such that the ring laser gyroscope may be dithered to have angular oscillations about a sensing axis.
U.S. Pat. No. 4,711,575 to Butler, entitled "Non-pendulous Counter-Balance Dither Mechanism For Laser Gyro", discloses a laser gyro dither mechanism utilizing a three spring suspension system.
U.S. Pat. No. 4,710,027 to Fersht, et al., entitled "Method and Apparatus for Mechanical Dither Stabilization Of A Laser Angular Sensor", discloses an apparatus whereby one, two, three or four dither mechanisms selectively may be simultaneously energized. The '027 patent is directed to individual dithering means connected to each of at least three ring laser gyros which are selectively energized.
U.S. Pat. No. 4,653,918 to Stjern, et al., entitled "Low Q Body-Dithered Laser Gyro Assembly", discloses a low Q ring laser gyro dither motor assembly.
U.S. Pat. No. 4,597,667 to Curby, et al., entitled "Dither Controller For Ring Laser Angular Rotation Sensor" discloses a dithering apparatus which is inserted into the hub of a ring laser gyro. The mechanism includes a pickoff sensor to measure parameters relating the sensor body position to a support means.
U.S. Pat. No. 4,349,183 to Wirt, et al., entitled "Spring For A Ring Laser Gyro Dither Mechanism", discloses a flexure spring assembly for a ring laser gyro dither mechanism using flexure springs between the hub and the rim. Each spring is driven by four piezoelectric crystal wafers.
U.S. Pat. No. 4,309,107 to McMere, et al., entitled "Laser Gyro Dither Mechanism", discloses a laser gyro dither mechanism utilizing a three-spring suspension system.
The dither motor reed apparatus provided by the instant invention offers significant advantages over the prior art schemes. One advantage is that the invention allows the fabrication of inertial measurement units in a package volume which is smaller than the packaging volume generally available with prior art schemes and which can withstand extreme shock.